1. Field of the Invention
This invention relates to electrochemical cells such as electrolytic cells having close-spaced, or "zero-gap", clearance between the permselective membrane and the electrodes.
2. Description of the Prior Art
In electrochemical cells having permselective membranes, particularly in electrolytic cells for the production of an alkali metal hydroxide and halogen, high concentrations of alkakli metal hydroxides are obtainable. Production of such concentrated solutions of an alkali metal hydroxide require high cell voltages and thus result in increased power costs from operating the electrolytic cells.
One solution to the problem of reducing the cost of power for operating electrolytic cells has been to decrease the clearance between the permselective membrane and the electrodes in order to decrease the ohmic losses through the electrolytes. When this spacing has been reduced almost to zero in close-spaced, or "zero-gap", electrolytic cells, it has been found that the release of hydrogen and halogen bubbles is impeded, especially the release of hydrogen gas at the cathode, as the bubbles of hydrogen are retained in the cathode-permselective membrane gap area. The presence of hydrogen bubbles in the gap area raises the cell voltage requirement and thus reduces the energy efficiency of the cell.
One prior art solution to the problem of increased cell voltage requirements under these conditions is the placing of the permselective membrane on the cathode of the electrolytic cell. When this is done, little or no space remains between the permselective membrane and the cathode. It has been found that this arrangement also impedes the release of hydrogen bubbles which are formed at the cathode and therefore raises the cell voltage requirement. It has therefore been found to be more desirable to provide a space between the cathode and the permselective membrane which is sufficient to permit the release of hydrogen gas bubbles. At the anode, halogen is released so that generally a sufficient space is necessary between the anode and the permselective membrane in order to avoid raising the cell voltage requirements.
In U.S. Pat. No. 4,105,514, a spacing means is disclosed which is placed between the anode and the permselective membrane of a wide gap electrolytic cell. The space between the cathodes and the permselective membrane is equal to or greater than the space between the anode and the permselective membrane and is free of a spacing means in order to provide maximum release of hydrogen gas at the membrane-cathode gap. In this patent, the spacing means is disclosed as a screen, or net, composed of any non-conducting chlorine-resistant material, such as a glass fiber, asbestos filaments, or a plastic material such as polyvinylidene chloride. Utilizing this spacing means between only the anode and the permselective membrane of the electrolytic cell allows an increase in the usual cathode-permselective membrane gap with a decrease in the cell voltage since "gas blinding" at the electrodes is avoided. The use of a spacing means in combination with a low to moderate differential pressure between the cathode compartment and the anode compartment of the cell results in increased energy efficiency.
In U.S. Pat. No. 4,135,996 there is disclosed a composite ion exchange membrane, including a cation-exchange membrane, having joined to it a microporous layer which is preferably at least as thick as the cation exchange membrane itself. When used in electrolysis of an alkali metal brine to produce chlorine and sodium hydroxide, these products are obtained with a high Faradaic yield. The microporous layer can be composed of asbestos or polytetrafluoroethylene.
In U.S. Pat. No. 3,438,879, there is disclosed a low energy efficiency, wide gap electrolytic cell having a chemically resistant, porous, protective barrier between the anode and one of the permselective membranes of the cell. This protective barrier can be composed of asbestos, glass fibers, or a synthetic polymer such as polyethyelene and polyvinylidene chloride. This protective barrier is said to extend the life of the permselective membrane.